1. Field of the Invention
The present invention relates to a novel pipe for a steam turbine, which is formed of a centrifugal casting pipe having high detection accuracy by an ultrasonic flaw detection test, and a manufacturing process of the novel pipe. The present invention also relates to a main steam pipe and a reheat pipe for a steam turbine, which are manufactured by employing the above process, and a steam turbine power plant using those pipes.
2. Description of the Related Art
In a steam turbine power plant, an operating life of the plant is affected, in particular, by soundness of a main steam pipe exposed to main steam among pipes exposed to high-temperature and high-pressure steam. If there is any defect inside the pipe, corrosion and/or cracks occur starting from the defect, thus resulting in a reduction of the operating life. In order to prevent troubles of damage and leakage during use, it is important to inspect the presence or absence of defects inside the pipe in the manufacturing and use of the pipe. Generally, a material test is carried out as a non-destructive test.
Non-destructive tests include, e.g., an ultrasonic flaw detection test (UT), a radiation flaw detection test (RT), a magnetic-powder flaw detection test (MT) which are specified in and carried out in conformity with JIS G0582, JIS G0581 and JIS G0565, respectively. Among them, the ultrasonic flaw detection test (UT) is advantageous from the viewpoint of accuracy in detecting the position and size of an internal defect.
Pipe materials are made of carbon steel, low-alloy steel, and a high-Cr alloy depending on temperature in use. At steam temperature of not lower than 550° C. corresponding to a target level of development and practical use which have recently been progressed with intent to increase temperature and pressure of steam from the viewpoint of energy saving, high-Cr martensitic steel is used which contains 8-13% of Cr and has high resistance against temperature and environment. As examples of alloy composition, 9%-Cr forged steel and 12%-Cr forged steel are specified respectively in Ka-SFVAF28 and Ka-SUS410J3 in technical standards for thermal power generation equipment. The alloy compositions of those forged steels are disclosed in, e.g., Patent Document 1 (JP,A 59-116360) and Patent Document 2 (JP,A 2-290950).